Zinc oxide (ZnO) materials typically have characteristics that are attractive for use in (i) optoelectronic semiconductor devices, which require emission or detection of electromagnetic radiation, (ii) high frequency and transparent transistors and (iii) radiation hardened devices, for example. However, the use of zinc oxide technologies has frequently been limited by a relative difficulty in reliably fabricating p-type zinc oxide materials with sufficiently high concentrations of relatively shallow acceptor impurities operating as p-type dopants.
Several attempts have been made to fabricate p-type zinc oxide materials using nitrogen (N) as a p-type dopant. Some of these attempts are disclosed in U.S. Patent Publication No. 2005/0170971 to Yata et al., entitled “P-type Zinc Oxide Semiconductor Film and Process for Preparation Thereof,” and U.S. Pat. No. 6,908,782 to Yan et al., entitled “High Carrier Concentration P-type Transparent Conducting Oxide Films.” Unfortunately, these attempts may result in zinc oxide films having only a relatively small percentage of activated p-type dopants at sufficiently shallow levels. Moreover, nitrogen may only have a limited solid solubility within zinc oxide, which operates as a limit on nitrogen activation and a limit on achieving uniform p-type dopant concentrations throughout a zinc oxide film.
Still further attempts have been made to incorporate phosphorus (P) and arsenic (As) as p-type dopants within zinc oxide films. Some of these attempts are disclosed in U.S. Patent Publication No. 2005/0285138 to Burgener et al., entitled “Persistent P-type Group II-VI Semiconductors,” and U.S. Pat. No. 6,291,085 to White et al., entitled “Zinc Oxide Films Containing P-type Dopant and Process for Preparing Same.” However, arsenic and phosphorus typically have poor acceptor ionization within zinc oxide, which may result from relatively high degrees of lattice relaxation at oxygen lattice sites. This relative high degree of relaxation typically causes these dopants to act as deep acceptors rather than shallow acceptors. Moreover, because acceptor conductivity within arsenic and/or phosphorus-doped zinc oxide devices may occur as a result of electrical coupling between these dopants and native point defects, which have concentrations that are highly dependent on temperature and variations in process conditions, these devices may be thermally unstable and have characteristics that cannot be reliably reproduced. To address these problems, post-growth annealing/activation steps have been considered to improve the electrical characteristics of the zinc oxide films. Such steps are disclosed in U.S. Pat. No. 6,852,623 to Park et al., entitled “Method for Manufacturing Zinc Oxide Semiconductors.”
Zinc oxide films can be fabricated using conventional vapor phase epitaxy (VPE) techniques, which include molecular beam epitaxy (MBE) and metal organic vapor phase epitaxy (MOVPE), and chemical vapor deposition (CVD) techniques. As disclosed in U.S. Pat. No. 6,887,736 and U.S. Patent Publication No. 2005/0020035 to Nause et al., incorporation of acceptor dopants such as arsenic, phosphorous and nitrogen can also occur in-situ. Unfortunately, because arsenic, phosphorous and nitrogen have relatively limited thermodynamic solubility in zinc oxide, only relatively low levels of electrically active acceptor impurities can typically be obtained. Furthermore, not all elements from Groups IA, IB, VA and VB of the periodic table of the elements ionize as shallow acceptors in zinc oxide. For example, some Group IA elements such as sodium and cesium appear to ionize as donors, while other Group IA elements such as rubidium and francium do not appear to contribute electromigratable charge carriers. In addition, copper, which is a Group IB element, appears to ionize isoelectronically as a deep acceptor with localized holes that may not electromigrate or drift in the presence of an electric field as required in practical semiconductor devices. It also appears that possibly all Group VB elements, including vanadium, niobium and tantalum ionize as donors as well.
Zinc oxide films fabricated using conventional processing techniques, such as molecular beam epitaxy (MBE), vapor phase epitaxy (VPE) and metal organic vapor phase epitaxy (MOVPE) may also include crystals that are oxygen deficient resulting from the relatively low partial pressure of oxygen during fabrication. As will be understood by those skilled in the art, this oxygen deficiency may result in the compensation of introduced acceptor ions and thereby limit the concentration of p-type dopant species that contribute to conduction. Conventional MOVPE processing of zinc oxide may also be severely influenced by a relatively high degree of reactivity between oxygen and zinc atoms. This high degree of reactivity may result in the deposition of three dimensional crystals as opposed to more desirable two-dimensional crystals having characteristics that are more compatible with microelectronic devices. Accordingly, conventional MOVPE processing techniques may result in the fabrication of zinc oxide films having a high level of lattice dislocations and grain boundaries that impede device performance by operating as gettering cites for acceptors.
Additional attempts to achieve acceptor conductivities in zinc oxide also include co-doping with lithium ions to achieve highly compensated substrates and then introducing acceptor impurities as described in the aforementioned Yata et al. application, or co-doping with gallium and nitrogen impurities as described in U.S. Pat. No. 6,733,895 to Kadota et al. However, co-doping may be unsuitable for semiconductor fabrication processes because different diffusion mechanisms may exist for each acceptor impurity on the zinc and/or oxygen sublattice, which can lead to thermal instability of at least one of the impurities. For example, in the case of lithium introduced as a compensating impurity in p-type zinc oxide realized through the incorporation of nitrogen, the solubility of nitrogen is reduced as the concentration of lithium increases. This variation in solubility can lead to the dominance of lithium, which is a deep acceptor, on the electrical characteristics of the zinc oxide.